Primate stress research has focused on severe social stress and its pathogenic effects on brain and behavior development. Consequently, few researchers have examined the possibility that mild forms of early social stress promote adaptive outcomes in primate development. Recently, our lab reported that brief, intermittent maternal separations (IS) in squirrel monkey infants subsequently lead to diminished anxiety, prosocial tendencies, enhanced glucocorticoid negative-feedback sensitivity, better spatial memory, and enlarged prefrontal brain regions. Since submitting my original NRSA proposal, I have determined that IS alters mother-infant social behavior at reunions. Also, in an unfamiliar environment, IS infants spend less time clinging to their mothers and more time exploring novel objects than NS infants. Although these data provide compelling evidence that IS triggers adaptations in postnatal development, it is not yet known a) whether IS produces differences in stress hormone responses and behavioral performance on a prefrontal-dependent cognitive task, b) whether the prefrontal enlargement previously identified in adult IS monkeys emerges earlier in postnatal brain development, and c) whether IS produces differences in prefrontal white matter integrity. This proposal delineates a series of experiments designed to answer these questions. Results from these studies will advance our understanding of how mild stressful experiences promote cognitive, emotional, and postnatal brain development. This animal model provides valuable insights on a basic process in human development, insofar as early emotional challenges foster adaptations useful later in life.